President Obama put on hold Saturday a plan to attack Syria for its alleged use of chemical weapons, arguing that the United States had a moral responsibility to respond forcefully but would not do so until Congress has a chance to vote on the use of military force.

The announcement puts off a cruise missile strike that had appeared imminent, a prospect that had the region on edge and stoked intense debate in the United States, where many dread getting dragged into a new war.

Obama did not indicate what he would do if Congress rejects the measure.

Lawmakers are scheduled to return from recess on Sept. 9to begin what is sure to be a contentious debate about the risks of injecting the United States into a conflict in which it has few reliable allies and enemies on both sides of the front lines. The Senate will hold committee hearings on the proposed strike this week, Majority Leader Harry M. Reid (D-Nev.) announced Saturday.

The decision to seek congressional approval for what the administration has said would be a short, limited engagement was a remarkable turn one day after Secretary of State John F. Kerry delivered an almost-prosecutorial case for military intervention. Obama made the decision Friday night following days of agonizing deliberations with members of his Cabinet, according to administration officials.

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Shifting the burden to Congress potentially gives the president a way out of the political bind he created last year when he said Syrian President Bashar al-Assad’s use of chemical weapons would be a “red line” for the United States. It also buys the administration time to shore up domestic and international support for a strike that many came to see as a hasty response with potentially catastrophic consequences.

The decision could work in Assad’s favor, giving him more time to prepare for an attack that could ultimately become politically untenable for Obama.

Obama argued Saturday that the United States would be setting a dangerous precedent if it did not respond to the Aug. 21 attack in a Damascus suburb, which U.S. intelligence officials say killed nearly 1,500 civilians, including 426 children.

“This attack is an assault on human dignity,” Obama said in an impassioned afternoon address in the White House Rose Garden. “It also presents a serious danger to our national security. . . . It could lead to escalating use of chemical weapons, or their proliferation to terrorist groups who would do our people harm.”

Some members of Congress applauded Obama’s move, a strikingly unusual one in presidential history, particularly for a leader who has been criticized for dodging congressional oversight. The president does not need congressional approval for limited military interventions, and the executive branch has not sought it in the past.

“At this point in our country’s history, this is absolutely the right decision, and I look forward to seeing what the administration brings forward and to a vigorous debate on this important authorization,” said Sen. Bob Corker (R-Tenn.).

Others chided the president, saying he was setting a troubling precedent on the presidential prerogative to use of military force.

“President Obama is abdicating his responsibility as commander-in-chief and undermining the authority of future presidents,” said Rep. Peter T. King (R-N.Y.), who favors a military strike. “The president doesn’t need 535 members of Congress to enforce his own red line.”

A measure authorizing a strike would seem to have a good chance of passing in the Senate, where the Democratic majority will likely be backed by influential Republicans who favor U.S. military intervention. It is expected to be a tougher sell in the House, where both war-weary liberals and conservative and libertarian Republicans have argued that the best of bad options is to avoid the temptation to use military force. It was difficult to predict on Saturday how the debate might unfold.

“Here’s my question for every member of Congress and every member of the global community: What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?” Obama asked.

The president said it would be seen a green light for adversaries seeking to build nuclear weapons and terrorist groups that got access to biological weapons. “Make no mistake — this has implications beyond chemical warfare,” he said.

After telegraphing in recent days that an attack with Tomahawk cruise missiles launched from U.S. Navy destroyers in the eastern Mediterranean was imminent, Obama sought Saturday to make the case that time is not of the essence.

Obama said the Pentagon is “prepared to strike whenever we choose,” but the formal decision was “not time-sensitive” and could come “tomorrow, or next week or one month from now.” Officials have said they planned an intense, short barrage that they hoped would deter Assad from using chemical weapons again.

Delaying the attack will allow Obama to travel to St. Petersburg for a Group of 20 meeting next week without the military operation dominating the news. Russia continues to back Assad and has warned against a U.S. strike.

As the congressional debate unfolds, the administration is likely to continue to seek support abroad. After the British parliament voted last week against intervention and the Arab League refused to back a strike, an administration that has placed a high premium on international coalitions found itself virtually alone, with only France offering to participate.

“I think part of this is the president started to read the tea leaves: There isn’t a lot of support for this, growing questions from Congress, and questions about our Syria policy that should have been answered a long time ago,” said Juan C. Zarate, a senior analyst at the Center for Strategic and International Studies who served as a top adviser to President George W. Bush. “With all that, he grew to doubt our ability to act with legitimacy.”

Administration officials have struggled to answer a fundamental question: If the strike was not aimed at chemical weapons sites — because hitting them could result in extensive collateral damage — what was the military objective of targeting other sites? Syria’s alleged chemical attack also added urgency to the policy debate over whether the United States should be seeking Assad’s ouster even though Islamist rebels affiliated with al-Qaeda form a major part of the opposition forces.

After the president’s speech, senior administration officials began a new push to garner support for military action. Kerry called Ahmad al-Jarba, president of the Syrian Opposition Coalition, to “underscore President Obama’s commitment to holding the Assad regime accountable for its chemical weapons attack,” the State Department said in a statement. Kerry also spoke with his counterparts in Japan and Saudi Arabia.

Defense Secretary Chuck Hagel and Gen. Martin E. Dempsey, the Chairman of the Joint Chiefs of Staff, were scheduled to speak to lawmakers Saturday.

The White House did not detail the scope of the request for authorization of military force it will submit to Congress, but there was no sign it is weighing more robust action.

“I know well that we are weary of war,” Obama said. “The American people have the good sense to know we cannot resolve the underlying conflict in Syria with our military.”

The ghosts of the U.S. war in Iraq war a decade ago have loomed large in the debate over whether to attack Syria. Iraq’s ambassador to Washington, Lukman Faily, said his country will not support a strike, mindful of the U.S. military’s troubled legacy in his country.

“We as a government and a people are not advocating a military solution,” Faily said. “We as a people have seen the results of such strikes without a clarity of the day after.”

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